I been trying to make a simple interface for an image processing task using OpenCV with C++ using Qt for the GUI. I'm able to load the image through the GUI but when I press the pushbutton_3, to convert the image to grayscale gives an error regarding OpenCV. I'm sure I'm doing something wrong. Can some one give me a help?

Please see below the files:

Qt Code: 
  1. //mainwindow.h
  2.  
  3. #ifndef MAINWINDOW_H
  4. #define MAINWINDOW_H
  5.  
  6. #include <QMainWindow>
  7. #include <QFileDialog>
  8. #include <opencv2/core/core.hpp>
  9. #include <opencv2/highgui/highgui.hpp>
  10. #include <vector>
  11.  
  12. #include <QtCore/QCoreApplication>
  13. #include <opencv2/core/core.hpp>
  14. #include <opencv2/highgui/highgui.hpp>
  15. #include <opencv2/core/types_c.h>
  16. #include <opencv2/imgproc/imgproc.hpp>
  17.  
  18.  
  19. namespace Ui {
  20. class MainWindow;
  21. }
  22.  
  23. class MainWindow : public QMainWindow
  24. {
  25.     Q_OBJECT
  26.  
  27. public:
  28.     explicit MainWindow(QWidget *parent = 0);
  29.     ~MainWindow();
  30.  
  31. private slots:
  32.     void on_pushButton_clicked();
  33.  
  34.     void on_pushButton_2_clicked();
  35.  
  36.     void on_pushButton_3_clicked();
  37.  
  38.  
  39.  
  40.  
  41. private:
  42.     Ui::MainWindow *ui;
  43.     //Images variables
  44.     cv::Mat image_Idl;
  45.     cv::Mat image_Lit;
  46.     cv::Mat image_Idl_G;
  47.     cv::Mat image_Lit_G;
  48.     double threshHold;
  49.  
  50. };
  51.  
  52. #endif // MAINWINDOW_H
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Qt Code: 
  1. //mainwindow.cpp
  2. #include "mainwindow.h"
  3. #include "ui_mainwindow.h"
  4. #include <iostream>
  5. #include <QtCore/QCoreApplication>
  6. #include <opencv2/core/core.hpp>
  7. #include <opencv2/highgui/highgui.hpp>
  8. #include <opencv2/core/types_c.h>
  9. #include <opencv2/imgproc/imgproc.hpp>
  10. #include <QSpinBox>
  11. #include <QSlider>
  12. #include <stdio.h>
  13. #include <stdlib.h>
  14. #include <vector>
  15.  
  16. using namespace std;
  17.  
  18.  
  19.  
  20. MainWindow::MainWindow(QWidget *parent) :
  21.     QMainWindow(parent),
  22.     ui(new Ui::MainWindow)
  23. {
  24.     ui->setupUi(this);
  25.  
  26.     ui->horizontalSlider->setRange(0,255);
  27.     ui->spinBox->setRange(0,255);
  28.  
  29.     connect(ui->horizontalSlider,SIGNAL(valueChanged(int)),ui->spinBox,SLOT(setValue(int)));
  30.     connect(ui->spinBox,SIGNAL(valueChanged(int)),ui->horizontalSlider,SLOT(setValue(int)));
  31.  
  32. }
  33.  
  34. MainWindow::~MainWindow()
  35. {
  36.     delete ui;
  37. }
  38.  
  39.  
  40. void MainWindow::on_pushButton_clicked()
  41. {
  42.     QString fileName = QFileDialog::getOpenFileName(this,tr("Load Lit Image"),".",tr("Image Files (*.png *.jpg *.jpeg *.bmp)"));
  43.     image_Lit = cv::imread(fileName.toAscii().data());
  44.     cv::namedWindow("Lit Image");
  45.     cv::imshow("Lit Image", image_Lit);
  46. }
  47.  
  48. void MainWindow::on_pushButton_2_clicked()
  49. {
  50.     QString fileName = QFileDialog::getOpenFileName(this,tr("Load Lit Image"),".",tr("Image Files (*.png *.jpg *.jpeg *.bmp)"));
  51.     image_Idl = cv::imread(fileName.toAscii().data());
  52.     cv::namedWindow("Ideal Lit");
  53.     cv::imshow("Ideal Lit", image_Idl);
  54.  
  55. }
  56.  
  57.  
  58.  
  59.  
  60.  
  61.  
  62. void MainWindow::on_pushButton_3_clicked()
  63. {
  64.     //Converstions
  65.  
  66.  
  67.     //Convert Lit to gray
  68.  
  69.  
  70.  
  71.         cv::cvtColor(image_Lit, image_Lit_G,CV_RGB2GRAY);
  72.  
  73.     //Convert Ideal gray
  74.  
  75.  
  76.         cv::cvtColor(image_Idl, image_Idl_G,CV_RGB2GRAY);
  77.  
  78.  
  79.       //Threshold the Images to a designated value
  80.                 // Lit
  81.  
  82.         threshHold = ui->horizontalSlider->value();
  83.  
  84.           cv::threshold(image_Lit_G,image_Lit_G, threshHold,255,cv::THRESH_BINARY);
  85.           cv::namedWindow("Gray Scaled Image");
  86.           cv::imshow("Gray Scaled Image", image_Lit_G);
  87.  
  88.  
  89. }
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qt_GUI_homo.jpg


Error from the compiler:

Invalid parameter passed to C runtime function. Invalid parameter passed to C runtime function. OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor, file C:\OpenCV\modules\imgproc\src\color.cpp, line 2834 The program has unexpectedly finished.